1. Field of the Invention
The present invention relates to a tunable filter preferably for use as a passband filter in a communication system, and in more detail, relates to a tunable filter including an elastic wave resonator.
2. Description of the Related Art
Occasionally, in a passband filter used in a communication system, a pass band is desired to be adjustable. Various passband filters, such as tunable filters, which satisfy this requirement, have been proposed.
For example, Japanese Unexamined Patent Application Publication No. 2005-217852 discloses a tunable filter that uses a plurality of surface acoustic wave resonators and a plurality of variable capacitors. FIG. 46 is a circuit diagram of the tunable filter described in Japanese Unexamined Patent Application Publication No. 2005-217852.
In a tunable filter 1101, a plurality of series arm resonators 1104 and 1105 are connected in series to a series arm that connects an input end portion 1102 and an output end portion 1103 to each other. In addition, parallel arm resonators 1106 and 1107 are respectively connected to a plurality of parallel arms provided between the series arm and a ground potential. The series arm resonators 1104 and 1105 and the parallel arm resonators 1106 and 1107 are defined by surface acoustic wave resonators.
A ladder filter circuit, which includes the series arm resonators 1104 and 1105 and the parallel arm resonators 1106 and 1107, is provided. Furthermore, so as to allow the pass band to be adjusted, variable capacitors 1108 to 1115 are connected. The variable capacitor 1108 is connected in parallel to the series arm resonator 1104, and the variable capacitor 1110 is connected in series to the series arm resonator 1104 and the variable capacitor 1108. In the same way, the variable capacitor 1109 is also connected in parallel to the series arm resonator 1105, and the variable capacitor 1111 is connected in series to the series arm resonator 1105.
Also in the parallel arm, the variable capacitor 1112 is connected in parallel to the parallel arm resonator 1106, and the variable capacitor 1114 is connected in series to the parallel arm resonator 1106 and the variable capacitor 1112. In the same way, the variable capacitor 1113 is connected in parallel to the parallel arm resonator 1107, and the variable capacitor 1115 is connected in series to the parallel arm resonator 1107.
In the tunable filter 1101, it is possible to increase a resonant frequency FrS of a circuit portion of the series arm with a decrease in the capacitances of the variable capacitors 1110 and 1111, such as the series capacitances. In addition, it is possible to decrease an anti-resonant frequency FaS of the series arm with an increase in the parallel capacitance, such as an electrostatic capacitance due to the variable capacitors 1108 and 1109.
In the same way, it is also possible to change a resonant frequency FrP and an anti-resonant frequency FaP of a circuit portion of the parallel arms, by changing the capacitances of the variable capacitors 1112 and 1113 connected in parallel and the capacitances of the variable capacitors 1114 and 1115 connected in series. Accordingly, it is possible to change the center frequency of the entire tunable filter 1101, by changing the capacitances of the variable capacitors 1108 to 1115.
However, in the tunable filter 1101 described in Japanese Unexamined Patent Application Publication No. 2005-217852, there has occurred a problem that the surface acoustic wave resonators used for the series arm resonators 1104 and 1105 and the parallel arm resonators 1106 and 1107 have small electromechanical coupling coefficients and the absolute value of a frequency-temperature coefficient TCF is small. In addition, the specific combination of the frequency characteristics of the parallel arm resonators and the series arm resonators has not been described.